auto-redirect-root-level-posts.php

<?php

/*
 * Plugin Name:  Auto-Redirect Root-Level Posts
 * Plugin URI:   https://gist.github.com/wpscholar/d37736b1b9c2439acf2289364e210bc5
 * Description:  Automatically redirects root level posts (e.g. /my-blog-post) to the new location (e.g. /blog/my-blog-post). Use anytime you update your post permalink structure from root-level to use a subdirectory prefix.
 * Version:      1.0
 * Author:       Micah Wood
 * Author URI:   https://wpscholar.com
 * License:      GPL2
 * License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 */

add_action( 'template_redirect', function () {
	/**
	 * @var WP $wp
	 */
	global $wp;
	/**
	 * @var WP_Query $wp_query
	 */
	global $wp_query;

	if ( $wp_query->is_404() ) {
		$query = new WP_Query( array(
			'post_type'      => 'post',
			'post_status'    => 'publish',
			'name'           => $wp->request,
			'posts_per_page' => 1,
			'nopaging'       => true,
			'no_found_rows'  => true,
		) );
		if ( $query->post_count === 1 ) {
			$redirect_url = add_query_arg( $wp->query_string, '', get_the_permalink( $query->post ) );
			wp_safe_redirect( $redirect_url, 301 );
		}
	}
} );